This invention relates generally to gain adjustable systems and more particularly to gain adjustable sigma delta modulator systems.
As is known in art, gain adjustable systems have a wide range of application. One such application is as in a multiplying circuit, such as in power measuring. Many such systems use digital circuitry in such application. One application is for use in watt-hour meters. One example of a watt-hour meter that multiplies two one-bit digital data streams is disclosed in European Patent Application 90313050 to The General Electric Company. One of the two one-bit digital data streams is representative of the current supplied to a load and the other of the two one-bit digital data streams is representative of the voltage across the load. In the disclosed watt-hour meter, an accumulator is used to accomplish the multiplication of the two data streams and to generate an output signal having a pulse rate that is representative of the power supplied to the load.
Another example of a power meter that multiplies two time varying signals is disclosed in an article entitled "A Power Meter ASIC With a Sigma-Delta-Based Multiplying ADC" by F. Op 't Eynde, published in the ISSCC94 Proceedings, paper TP 11.1. In the system disclosed by Eynde, a one-bit data stream from a sigma delta modulator is multiplied in the analog domain with a second input time varying signal. The resulting product is digitized using a second sigma-delta modulator. Since the multiplication in the Eynde system is accomplished in the analog domain, the system suffers from the same drawbacks as the analog multipliers discussed above.